Poplar is one of the softest/lightest hardwoods, ranking down with Basswood. On the plus side, it's pretty smooth stuff - kinda like Alder. Doesn't require as much prep to finish. It's normally finished opaque, as it often has green streaks that would interfere with most transparent finishes.
Swamp Ash, which is just White Ash that's been grown in a swamp, is much harder and denser, although it has a fairly open grain similar to Oak. Pain in the shorts to put a fine finish on, as it will require some grain filling.
If you ever make it into a Lowe's, Menard's or Home Depot, they will typically have milled Poplar boards for sale. You can see/feel the stuff there. Compare it to the Oak boards they have, and other than the Oak being the wrong color and too uniform, you'll get a good feel for the difference between Poplar and Ash.